There are three reactions to the concept of “feeding your plants”:  

1) Plants feed themselves through Photosynthesis (you don’t need to feed your plants!) 

2) They’ll pick up elements in the soil that they need for growth and development, nobody feeds the plants in a native forest. 

3) What’s scientifically needed for our gardens is a soil test that indicates which chemical elements are lacking in soil. 

This is about the Science of Fertilisers. 

This is the time of the year when soil temperatures are great for plant growth. Roots work over-time to extract minerals, dissolve them in water and transport them through the phloem bundles to the leaves of plants, where photosynthesis puts it all together and creates carbohydrates and chemicals that allow cell-elongation (growth).  

Plants use three main elements for bulk growth “food”: Nitrogen, Phosphorous, and potassium (K), which are generally known as NPK (their chemical symbols).  

N is used to make the green stuff: leaves and chlorophyll (LAWNS, LETTUCE, SPINACH, HEDGES). 

P is good for root development and plant health (CARROTS, PARSNIPS, POTATOES, etc). 

K (potash) is for sex: flowers and fruits (TOMATOES, APPLES, STRAWBERRIES, FLOWERING PLANTS). 

Other elements needed for plant functioning are needed in much smaller quantities:  

Mg (Magnesium), S (Sulphur), Ca (Calcium), Cu (Copper), Fe (Iron), Zn (Zinc), Mn (Manganese), and a heap more of those minor “trace elements”. 

There are a few different types of fertilisers:  

1) “General” Fertiliser – for general growth: usually something like N-P-K 7-3-6 or 12-4-13 (note how Phosphorus is usually less than the N and K). 

2) Potato fertiliser: N-P-K 3-9-6, also good for carrots and parsnips. 

3) Tomato or rose fertiliser - usually higher in potash (K): N-P-K 3-4-9. This helps to stimulate flower and fruit growth. 

Of course, you can always use the general fertiliser (which tends to be highest in Nitrogen) and simply add a few handfuls of Superphosphate (P) if you grow root crops, or handfuls of Sulphate of Potash (K) if you want to up the dose of K (potash) for flowers and fruit.  

Organic fertilisers usually have lower concentrations of elements, and they are often less prone to fertiliser run-off into water courses. Chicken poo (a “Natural” organic fertiliser) has a high content of Nitrogen which can burn plants – I would always send it through a cycle of composting before use. 

These are very general comments on how to use fertilisers; some plants require a bit more detailed knowledge or would benefit from changing fertiliser regimes in different times of the year (Cymbidium orchids have a green-growth phase to make leaves in spring and summer – followed by a flower bud initiation in autumn and flowering in winter).  

I use General fertiliser on my young tomatoes to get them growing up and create a strong climbing vine, before the flower buds are formed.  

Once they start flowering, I assist the plants with more potash to keep on making fruit – I just switch to a higher potash fertiliser such as Wet&Forget’s Seaweed Tea.  

Dog Pee fertiliser  

Most homeowners let their dogs use the backyard as their own personal toilet. Urine is very high in Nitrogen. Too much nitrogen will burn the grass and create yellow patches after sensational, dark green growth (a dog will usually come back to the same patch – territorial “marking”).  

The concentration of nitrogen in the dog’s urine depends on the type of dog, its sex, and what the animal eats. Larger dogs will pee more and cause more damage. Female dogs also tend to cause more damage than males because they squat and urinate in one concentrated patch whereas the males spray their urine over a larger area and in much smaller doses each time. Finally, diets high in protein can increase the concentration of nitrogen in the urine since protein breaks down to release nitrogen compounds. 

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